1. Field of the Invention
The present invention relates to a method for forming an azo colorant and a recording material that utilizes the same, and more specifically to a method for forming an azo colorant and a recording material which are capable of realizing a quick reaction rate and quick color development.
2. Description of the Related Art
A large number of reports have been presented concerning an azo colorant forming reaction using a coupler and a diazo compound. Representative literature includes “Color Chemistry” (written by Heinrich Zollinger, Weinheim New York Bose L Cambridge, 1987), “Synthetic Dye” (written by Hiroshi Horiguchi, Sankyo Shuppan Corp., 1970), and the like. Detailed explanations are given in sections concerning azo dyes and azo pigments.
Several methods for forming an azo colorant are known. An azo coupling reaction is commonly used. Azo coupling is a reaction which forms an azo colorant from a diazo compound and a coupling component (hereinafter referred to as a “coupler”). Below formulae of representative azo coupling reactions are shown. 
From the above examples it can be seen that in an azo coupling reaction, a counter anion (X−) leaves the diazo compound, and a hydrogen atom leaves the coupler, and an azo bond is formed.
Accordingly, most couplers have a hydrogen atom at a position at which the azo bond is generated, and there are only a few examples having a substituent thereat.
For example, in Helvetica Chimica Acta 55. 2139 (1972), a coupling reaction between a naphthol based coupler having a leaving group (halogen) at a coupling position and a p-chlorobenzene diazonium salt is reported. However, a coupling rate of a coupling reaction using a coupler having this leaving group is much lower than that of a coupling reaction using an unsubstituted coupler. 
Besides this report, it is generally understood that introduction of a substituent at the coupling position of a coupler result in a slow reaction rate in the azo coupling reaction. A diazo compound to be used is selected such that a combination of the coupler having a leaving group and the diazo compound has a high reactivity, for example, a diazo compound with an electron-attractive group, or a diazo compound in which a diazo group is bonded to a χ-electron-deficient heteroring skeleton is preferably used.
Also, in relation to photography using silver halides, couplers having leaving groups have been discussed widely, and a large number of reports have been provided (for example, Corona Corp., Revised Edition, “Basics of Photographic Engineering—Silver Salt Photography—”, and the like). However, the leaving groups in these reports were defined to be groups that can be released by reaction with an oxidized color developing agent such as quinonediimine. No examples of a compound having a group that can be released by reaction with a diazonium salt have been disclosed.
In a color-developing reaction that uses a coupler and a diazo compound, one important requirement is that a color-developing reaction rate (coupling rate) is high and there is little side reaction. In practical terms, when considering application to azo colorant synthesis, a recording material and the like, it is necessary to provide a certain greatness of reaction rate (speed) for the azo coupling. In general, in cases where the same diazo compound is used, the coupling rate is determined substantially by the coupler skeleton. It is possible to improve the coupling rate by appropriately changing substituents at positions other than the coupling position of the coupler. However, the coupler stability deteriorates, which causes various problems.
A diazo compound as described above forms an azo dye by reaction with a coupler such as a phenol derivative, and has photosensitivity so as to be decomposed by light irradiation and lose activity. Therefore, diazonium salt compounds have been used as light recording materials including diazo copys for a long time, as disclosed in the above-mentioned literature (“Basics of Photographic Engineering—Silver Salt Photography—”, edited by Japan Photography Association, Corona Corp. (1982), see pages 89 to 117 and 182 to 201).
By utilizing the characteristics of the diazo compound, which is decomposed by light and loses activity, the same has recently been applied to recording materials, requiring fixation of an image. As a representative example, a light-fixing type heat-sensitive recording material in which a diazonium salt compound and a coupler compound are heated according to an image signal for reaction so as to form an image, and then irradiated with light for fixing the image, has been proposed (Koji Sato et al., Imaging Electronics Association Journal, Vol. 11, No. 4 (1982), pages 290 to 296, and the like).
Even in the case of application in such a recording material, the same problem as in the above-mentioned azo colorant forming reaction occurs. That is, if the coupler activity is raised for improvement of the color development property of the recording material, the coupler stability is lowered, so that problems such as an increase of fogging in a base portion, generation of light-exposure coloring, and deterioration of image storability with respect to light and heat arise. Thus, coupler activity has an inverse relationship with coupler stability. Therefore, an effective method for dramatically improving the coupling activity without greatly deteriorating the coupler stability has not been found yet. In contrast, in cases of adopting a method for improving diazo compound activity, formation of capsules becomes difficult and fogging tends to be increased. Accordingly, it has been desired to obtain a recording material having excellent color-developing property and stability without impairing the stability of each compound related to color development.